Focusing-cap for telescopes.



H. G. MUSTIN.

FOUUSING GAP FOB. TELESUOPES.

APPLICATION 11,111) 11110.13, 1907.

Patented June 8, 1909.

2 SHEETS-SHEET 1.

A 4 I a O J 6 L; A?

Fi i i 5 1 i z 2 g i 1 e -=j F a wi flmeooao im'pgwtoz H. G. MUSTIN'.

rocusnw GAP FOR TELESGOPES.

APPLICATION FILED 1130.13, 1907. 9g% 4gg Patented June 8, 1909. 2 SHEETS-SHEET 2.

flllwwm W H I? W e wco M fi W" J1}? v- W J f J L .QWHUI, m Tr ,iiiilf a nNiiEn STATES PATENT 01m HENRY O. MUSTIN, OF THE UNITED STATES NAVY.

FOCUSING-GAP FOR TELESCOPES.

To all whom it may concern:

Be it known that I, HENRY C. MUSTIN, (lieutenant, U. S. N avy,) at present attached to the U. S. S. Kansas, League Island N avy- Yard, Philadelphia, Pennsylvania, have invented certain new and useful Improvements in Focusing-Caps for, Telescopes; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

Myinvention relates to focusing caps for sighting telescopes, and the object of my invention is to provide such a cap which will render the images of miniature targets when viewed at short distances, clear and distinct, although the telescope itself is focused for battle or for target practice.

It is well known that the vessels of our navy are equipped with practice apparatus designed to improve the marksmanship of those whose duty it is, or might be, to fire the guns in battle; and that said apparatus comprises a miniature target carried by the ship on what is known as Morris tube booms. But since this miniature target is quite close to the telescope, sometimes not more than thirty inches, it'is evident, that in order to clearly see the image of this miniature. target in the telescope, it is essential that the same be specially focused. In other words, whenever this practice outfit is to be used on ship board,.it is essentialthat the focus of the sighting telesco es be changed from that suitable for batfle or for target practice, to that suitable for this miniature target. This changing of the focus and sights on the guns, is objectionable for many reasons obvious to skilled gunners, and, as above'stated it is the object of my invention-to render the same unnecessary, and to therefore permit the telescopes to be permanently focused and the sights permanently adjusted for battle, or for target Practice.

To t ese ends my present invention con- Sists in a focusing ca provided with a air of lenses, adapted to e secured to the ob ect end of a telescope, and to render the image of the miniature target clear and distinct'although the sighting telescope remains focused for battle or for targetpractice.

Referring to the accompanying drawings forming a part of this specification :Figure Specification of Letters Patent.

Application filed December 13, 1907.

Patented June 8, 1,909

Serial No. 406,388.

nation of a plus anda minus lens to be em-- ployed in my cap, and Figs. 7, 8 and 9, further diagrams illustrating the principles involved in my invention.

In order that my invention may be clearly understood, it is necessagy to briefly outline its optical princi les as oll0ws:As is well known when a te escope is in focus for battle or for target practice, its object glass receives practically parallel pencils of rays of light, and when pointed at an object as near as the end of what is known in thenavy'as a Morris tube boom, the form of the entering pencils is divergent instead of parallel. Therefore, in this latter case, any image of an object at the end of said boom, instead of lying in the plane of the cross hairs, wouldlie considerably to the rear and would therefore appear blurred, producing both indistinctness anda large amount of parallax. It is evident that tice instead of using a single plus lens to render'parallel these entering divergent pencils,

it is preferable to use a combination of a plus and a minus lens, which as is well known, and as willfully appear below, is the full optioal-equivalent of a single plus lens. Such 1 a combmation is illustrated in the lenses illustrated in Figs. 8 and 9 of the drawings. But

not only in practice, is it necessary tp correct these divergent entering rays, but it is also mula where t is the height of the miniature target;

7 T the height of the service target, 1" is the mean or average target range, and f, the focal length of the focusing lens. This equation is independent of the magnifying power consideration.

of the telescope and the distance between the focusing cap lens and telescope, and in order to render clear the operation of my focusing cap it will be necessary to show how the same is derived. But before deriving this equation it will, also,.be necessary to make clear the meaning of certain planes and points used in deriving the same.

Referring to Fig. 3, of the drawings, if A and B are the centers of curvature of the surfaces of a double convex lens C, and if B D, and AE, are parallel, then a ray of lightH D, entering at D, will -be refracted along the path D E, cutting A B, at G, and finally emerging along E K, parallel to its original direction, all as is well known. If we now prolon HD, until it cuts A B, at M, and prolong E, until it cuts A B at L, and then draw the planes 0 P and R S, through the points L and M, respectively, and perpendicular to A B, the points L and M, will be unit points. That isto say, in the modern treatment of optical problems, the planes 0 P and R S, are called unit lanes, and the points L and M, are called unit points, and it is well known that in considerin a lens, or a combination of lenses, only 1; e cardinal planes consisting of the unit planes and the ocal planes (which are the planes perpendicular to the axis drawn through the principal foci) of said lenses need be taken into This will be apparent from an inspection of Fig. 3, which in the line D E shows the actual'path of the rayH D through the lens. It is clear that if this line D E had been omitted, and the actual path had been considered as assing from the entering oint D to the st unit point M, from the st unit point M to the second unit oint L, and from the second unit point L, a ong the line L E Kto its endfiarecisely the same parallel rays H D and E before and after emergence would have been obtained. In raphically indicating such a ath, it would only be. necessaryto know t e unit planes, the unit points and to simply draw.- the parallel straight lines H M an L K,

all as is wellknown. Therefore, in Figs. 4, 5, 6, 7,8and9, g

I will omit the actual paths of the rays through the lenses, and will only indicate graphically the paths of the rays in straight lines to the first unit point, thence in straight lines from the first unit point to the second unit point, and finally in straight lines from the second unit point out of the lens to the end of the path, or focus of said rays.

Now coming back to the question-of deriving the formula for expressing the height of the miniature target, and referring especially to Fig. 4, F F, and F F, represent the focal lanes passing through the principal foci o the telescope objective 0; P P and P P, the unit planes of the same, and F, the focal length of said objective. These unit planes are differently located from those in Fig. 3, owing to the faces of the lens being different. D, represents the service target of a height, T, above the axis of the telescope, and r, is the mean target practice range which of course is great in value as compared to the height T. Now in accordance with the well established properties of unit points, a straight line 1 from the upper extremity of the tar et D to the first unit point U, of the lens represents the entering path of a ray that passes through the obj ective without deviation; and a parallel straight line 2, drawn through the second unit point V, gives the final path of the ray.

This ray is the axis of a pencil, which being parallel before entering the objective C, is convergent after emergence from said objective and by the well known laws of optics, since the distance 1" is very great this pencil must come to a focus in the second focal plane F F". The point E, where the axis of this pencil intersects the second focal plane, isobviously the focus corresponding to-the upper extremity of the target D. The distance of this point below the axis of the telescope gives the height of the inverted image of the target to which I will assign the dimension '5. The boundary of this pencil is shown by broken lines 3 and 4 in Fig. 4. The actual path of the rays inside the lens is of course direct from the point of incidence to the point of emergence; but in the figure I have used the graphic method, as above stated, for the interior path which is based on the well known fact, that the original and final directions of any ray intercept equal distances on the unit planes. That is to say, the lines 1 and 2 intercept zero distances on the unit planes P P and P P, while. the lines 3 and 4 between said planes, are paral- 'lel to the axis of the lens and therefore intercept equal distances on said planes.

Fromv the figure by "a similarity of triangles Referring to Fig. 5, of the drawings, I

the plane of said target (1, as shown. In

' this figure the graphic path of a ray of light which passes through the lens without deviation 'is shown by the lines 5 and 6, which pass to the first unit oint U of the lens G,

thence to the secon unit point V of said lens, and finally from said oint V in a direction parallel to the first irection. These lines 5 and 6, represent the axis of a pencil that is divergent before entering the lens G, and is parallel after emergence, for the reason that by construction its source lies in the first focal plane f of the lens G, all as. is well known. ow if we draw from the first unit point U, of the lens C, in Fig. 5, the line 7, parallel to the line 6, cutting the unit plane p {1 of the lens G, in the point 8, then draw a ine parallel to the axis of the lens from the point 8, to the point 9, in the unit plane p p, and then draw the line 10, from the point 9, to the target, we have one boundary of the pencil -of rays whose axis is represented by the lines 5 and 6. Evident ythe lines 11 and 12, may be likewise shown to be another boundary of said pencil. But this pencil enters the lens C, in

the parallel form, like the pencil from the target D, illustrated in Fig. 4, and if we draw a line 13, from the second unit point V, of the lens C, arallel to the line 7, it must cut the focal p ane F F in the focus E, of said pencil, which will mark the extrernity of the first image corresponding to the upper extremity of the miniature tar et.

Having fixed the point E, the oints V and the line 14, are readily ound, and the other boundary of the convergent pencil,

as shown in Fig. 5.

The condition for an e ual apparent size of the service target, and of theminiature tarlget, requires that the height of the image in ig. 5, to be e ual to the height 'i, of the image in Fi 4. But from Fi 5, considering the es 6, and, the similarity of the triangleswe have;

3L2 ff But from Fig. 4, a

' i E F 1' Therefore I It is evident that if a single lens G, is used as above indicated, that my focusing caps would be good only for one'particular distance of the miniature target, or the slight variations which might "be covered by the length of travel provided for in the tube that holds the said lens. But it is'evident from well known optical principles that we can replace a single lens by acoaxial system that will have an equivalent focal length equal to the single lens. In other words, the lens G1, in Fig. 5, can be replaced by a combination of two lenses properly spaced in accordance with the well known law expressed by the formula,

Where f, is the equivalent focal length of the combination, f, is the focal length of the first lens and ,-is the focal length of the second lens, w ile K, is the distance the first focal plane of the second lens lies in front of the second focal plane of the first lens. It is evident, from this equation, that by varying the distance between the two lenses of this combination, we can vary the equivalent focal length of said combination.

In practice, on account of varyingdistances which are encountered, I find it most convenient to employ a combination conimmaterial in which order these lenses are placed, but in Fig. 6, the first G is shown positive and the second G negative. The cardinal planes of the first lens G are f ,f p, p, p p f f its focal length isf. The cardinal planes of the second lens, whose focal length i 1/ are; t/ f/Iz PM u n /1 f" f,. Since the first focal pane of the second lens is actually a distance K, in rearof the second focal plane of the first lens, we may say that the first focal lane of the second lenslies' forward of t e second focal plane of the first lens by a distance equal to K. The assignment of a ne ative sign to the value of a focal length 0 a divergent lens agrees with the customaryconvention in optlcs and is here used for convenience in the formulas. of the combination is, therefore,

Therefore, this combination'has convergent properties, and will give the same result as a single plus lens having a focal length f.

lent ocal length of the combination diminishes. In other words, as the first lens is moved outward the first-focal'plane of the combination moves inward. If the value of sisting of one plus and one minus lens. It is- The equivalent focal length and the value of this focal length is .positive. A

Furthermore, if the lenses are moved apart, '5. the uantity -K, increases and the equivaanother, until the first focal plane f 1 f of the lens G coincides the second focal plane f',,, of the lens G; for this position the uantity K, vanishes and the equivalent ocal length, as iven by the above equation, is evidently infinite.

Having found a suitable combination of lenses, to take the place of a single plus lens in my focusing cap, it still rema1 ns to determine the distance of the first focal lane of the combinationin front of the first ens; for this is the position in which the miniature target must be placed to give the desired result. This distance can be determined by applying the'well known relation between conjugate foci, expressed by the formula XY= (f) Where X, is the distance between the firstfocal plane and the object, Y, is the distance between the second focal plane and the image, and f, is the focal length of the lens. This will be made plainer from an inspection of Fig. 7, where g, represents a point on the axis of the lens G at a distance X, in front of the first focal planef f length f of this lens is indicated as in Fig. 6. Here we may find the image of the point g, at g, on the axis of the lens, by laying off a distance Y, in rear of the second focal plane f, f such that The unit planes of this lens bein found at p, p, and 10 p,; we may now p ot, by the graphic method, the .path of any ray from g, to the point q, as shown in broken lines in said figure, by following the principles above set forth.

Taking the diagram shown in Fig. 7, we will now add the cardinal planes of the negative lens G having a focal length-f", of a magnitude not greater than f, placing said lens so that its first focal plane passes through the point g, and we thus form the combination shown in Fig. 9, where the unit planes p rp i and i 21 2 lens G are illustrated, as well as the focal The second focal plane f f ofth1s lens G is omitted for the sake of clearness, but it is shown in Fig. 6.

From an inspection of Fig. 9, it will be seen that the ray which originated at the point g, and would have converged to the point now emerges from the lens G in'a direction parallel to the common axis of the lenses as indicated by the line 16. The reason why this'ray is parallel to the common axis of the lenses after emergence is, because before entrance to the negative lens it had a line of direction toward the first principal focus, which 'was the point q, where the first The focal space between the lenses.

of the negative that a ray starting from g, after emergence from the combination, is parallel to the common axis, and it is evident that the point q, isthe first principal focus of the combination. Therefore, if we make the distanceY K, the combination will be the same as that dis closed in Fig. 6. The distance of the first focal plane of the combination measured from the first unit plane of the first lens is d=f+X or since Referring back to Fig. 6, if we provide two lenses of e ual focal length of the class there shown, an with the faces arranged in the way indicated, the quantity K, will also be the distance between the contiguous faces. If we then neglect the small distance between the first face and first unit point of the first lens, we can say the first focal point of the combination lies at .a distance forward of the first lens amounting to its focal length plus its focal length squared and divided by the That is to say, if, for instance, we put f f, =7 inches andK= inch, then (2 =252 inches. This arrangement in a focusing cap would suit a miniature target 21 feet forward of the first lens. Then with this focusing cap, and given distance of the miniature target, we could determine the spacing of the lens. For instance, to suit a'target two feet from the first lens we would have whence-K=2.888 inches. This will probably be the extreme case for the motion of the lenses apart, for no miniature tar ets. are likely to be located closer to the o jective than 24 inches. Therefore, if the 'focusin cap be designed to permit an extension 0 2.888 inches with the lenses as above, it may be used for any distance of miniature targets, from two feet up to. infinity. a convenience to the battery oflicer, the focusing cap could be marked with a scale in one foot divisions. corresponding to distances of the miniature target from two feet up to 20- feet, which will probably include all conditions found inthe vessels of ournavy. This scale may be calculated from the formula;

But in view of the possible small errors in the curvature of the lenses, it would be preferable to mark this scale experimentally.

It is evident that if we substitute the equivalent focal length for f, in the equation 1' Now if the focal length of each lens is seven inches, the battery oflicer can easily calculate the size of the miniature target. In such calculation he should use the following formula, where all terms are in feet a l t=T(d- 12) In this formula,

t=the height ofthe miniature target,

T =the height of the service target,

r the mean target'practice range,

d=the reading ofthe scale on the tube in the focusing cap,or else the measured disfance of the mmi' 'ature-target from the first ens.

For convenience in explaining the focusing cap, I have used a plano-convex and a planoconcave lens, placing them with their curved sides opposing. But as regards spherical aberration this would -be a poor'disposition of the radii of curvature, and there would probably be a sensible distortion of the 1ma e. Although it isimpossible to. get rid of al the spherical aberration in an instrument where a focalle'ngth is variable, we mayimprove matters .by using the same struments in genera type of plus lens, placing its curved face to the front, and instead of using a plano-concave lens, we may use an equi-concave, as shown in Fig. 8, where the cardinal lanes, focal lengths and distance-K, are all indicated as in Fig. 6.

Another advantage of employing the disposition and kind of lenses shown in Fig. 8, is apparent on an inspection of the lens 12, t ere shown. Here the formulas for distance and size of the miniature target become exact; for the first face and first unit plane of the first lens 12, are co-incident. It 1s apparent, however, that we can no longer take K; as a distance between the opposing lens faces, for this would introduce two small errors. which is the distance between t e first focal plane of the second lens and the second focal plane of the first. Having arranged the lenses as shown in Fig. 8, it is then only necessary to mount the same in some convenient manner, as for example as shown in Fig. 2, and to provide for thelr ready attachment to the'telescope.

Referring now toFigs. 1 and 2; 1, represents the body of the telescope; 2, the end carrying the eyepiece;- and- 3, the end carrying the object lass. Over the end 3, of the telescope fits t e cap 4, which is screw threaded K, must be given its roper value,

on to the tube 5, which in turn slides-on the tube 6. 7, is a clamping band operated by the thumb screw 8, for holding the tube 5, in its adjusted position on the tube 6; and 9, represents a suitable scale preferably graduated for feet, and indicating the distance of I the miniature tar et from the object glass end 3, of the 'te escope. 10,, indicates a thumb screw for firmly clamping the cap 4, on the end of the telescope, and for this urpose the said cap is provided with split ugs 11, through which said thumb screw 10, asses. The plus lens employed in this comhination is designated 12, and the equi concave lens is designated by 13.

In operation the telescope is focused for battle and kept at all times so focused, and

' therefore ,readyfor use.,- Whenever it is desired to have sub-target practice, the focusing cap is secured in place as above described, on the objective end of the telescope, and-the sights are directed to the miniature target carried by'the ship, and which at once appears clear and distinct; and, also, of the same size. as a distant real target ordinarily ap ears when aimed at.

t is evident from the foregoing, that my invention is by no means limited to sighting telescopes, but is applicable to optical inwherever it is desired that the instrument shall produce a clear image of objects that are far and near, without adjusting the focus of the lenses. In fact my invention will be found useful to oculists. Therefore I do not wish to be vention.

What I claim is 1. In telescopes, the combination of the lenses thereof adapted to be focused for a distant target; and means causing said lenses when so focused and directed on a near miniature target, located at a plurality of distances from said telescope, to produce distlnct images of said miniature target, sub stantially as described.

2. In telescopes, the combination of the lenses thereof adapted to be focused for a distant target; and means comprising a plus lens causing said lenses when so focused and directed on a near miniature target, located at a plurality of distances from said telescope, to produce distinct images of said miniature target, substantially as described.-

3. The combination of a telescope, with means comprising a plus lens adapted to cause said telescope when focused for a distant target of a given size, and when directed on a near miniature target of a proportionate size and at a plurality of distances from said telescope, to produce distinct images of said miniature target and of the same size as the image of the distant target, substantially as described.

4. The combination of an optical.instru ment provided with lenses adapted to produce a distinct image of a distant object, of an attachment for said instrument adapted to cause the same, when focused for a distant target, to produce clear images of a near miniature target when directed on the same, and when located at a plurality of distances from said telescope, substantially as described.

5. In sighting telescopes, the combination of the lenses thereof, with an attachment comprising a plus lens, adapted to cause said telescope when focused for adistant target of a given size and directed on a near miniature target of a Iroportionate size located at a plurality of distances from said telescope, to produce distinct images of said miniature target and of the same size as the image of said distant target, substantially as described.

6. In sighting telescopes, the combination of the lenses thereof, with a focusing cap attachment adapted to fit the object lass end of said telescope, comprising a plus ens, and adapted to cause said telescope, when focused for a distant target and directed on a near miniature target located at a plurality ofdistances from said telescope, to produce distinct images of said near target, substantially as described.

7. In telescopes, the combination of the lenses thereof adapted to be focused for a'distant target; and means comprising plus and negative lenses causing said telescope lenses, when so focused, to produce distinct images of a near miniature target when directed on the same, and when said target is located at a plurality of distances from said telescope, substantially as described.

8. The combination of an opticalinstrument provided with lenses adapted to be focused for a distanttarget, of an attachment for said instrument, comprising plus and negative lenses, adapted to cause the instrument when focused" for said distant target and directed on a near miniature target, to

produce clear images of the latter when located at a plurality of distances from said telescope, substantially as described.

9. In sighting telescopes, the combination of the lenses thereof, with an attachment comprising a plus lens and a negative lens suitablv spaced apart, and adapted to cause said te escope, when focused for a distant target and directed upon a near miniature target located at a plurality of distances from said telescope, to produce distinct images of said -miniature target, substantially as described.

, 10. In sighting telescopes, the combination of the lenses thereof, with a focusing cap attachment adapted to fit the object lass end of said telescope, com rising a plus ens,

and a negative lens suita ly spaced apart, and adapted to cause said telescope, when focused on a distant target of given dimensions and directed to a near miniature tar et of proportionate dimensions and at a p u rality of distances from said telescope, to produce distinct images of said miniature target, and of the same size as the image of the distant target, substantially as descrlbed.

-11. In a sighting telescope adapted to be permanently focused for battle, or for target practice range, the combination with a focusing cap adapted to be attached to the object glass end of said telescope and provided with a lano-convex lens and an equi-concave lens, separated at such adistance that they will cause said telescope, while so focused, to produce distinct images of a near miniature target when directed on the same, and when the latter/is located at a plurality of distances from the telescope, substantially as described.

12. The combination of a sightin telescope adapted to produce an image 0 a distant target and to be left focused for battle, or for target practice ran e, and a focusing cap adapted to be attac ed to the object glass end of said telescope, provided with a plane-convex lens, and an equi-concave lens separated at such adistance that they will cause said telescope, while focused as above stated, and directed on a near miniature target of a size proportioned to said distant target, to produce a distinct image of said miniature' target and of substantially the same dimensions as the image of the distant target, substantially as described.

13. The combination of a sighting telescope adapted to produce an image of a distant target and to be left focused for battle, or'for target practice range, and a focusing cap ada ted to be attached to the object glass en of said telescope, provided with a combination of lenses consisting of a positive and a negative lens separated from each other, and having means adapted to so ad just the same that they will cause said telescope, while focused as above stated, and

directed on a near miniature target of a size proportioned to said distant-target, to produce a distinct image of said miniature target and of substantially the same dimensions as the image of the distant target, substantially as described.

14. A focusing cap for optical instruments provided with a positive and a negative lens,

adapted to be adjusted toward and from each other, and having means for attachment to an optical instrument, substantially as described.

15. A focusing cap for sighting telescopes comprising a tube having a negative lens secured thereto, a second tube adjustable with relation to said first tube, and having a positive lens secured thereto, and means for attachment to the object end of a telescope, substantially as described.

16. A focusing cap for telescopes comprising a tube having an equi-concave lens secured therein; a second tube sliding in the first tube and having a positive lens secured therein; 'a clamp to sec'ure said tubes and lenses in their adjusted positions, and-a second clamp for securing the cap to a telpsc ope, substantially as described.

In testimony whereof, I afiix my signature,

in presence of two Witnesses.

' HENRY C. MUSTIN,

l/Vitnesses: r

CORBINE M. MUsTIN, J. D. COPELAND. 

